The invention relates to a system for canceling interference in a receiver and internal interference in a time-division multiple access (TDMA) receiver.
Global System For Mobile (xe2x80x9cGSMxe2x80x9d) receivers typically derive many different clock frequencies from a single reference frequency and are susceptible to xe2x80x9cself-jammingxe2x80x9d or interference at particular frequency channels. For example, it is common to use a crystal oscillator as a system clock operating at a frequency of either 13 MHz or 19.5 MHz because use of such frequencies permits other clock frequencies to be easily derived. A problem arises when a harmonic of the system clock is the same frequency as one of the GSM communication channels. For example, the 72nd harmonic of a 13 MHz system clock has energy at 936 MHz (72*13=936). Channel of the GSM system is centered at 936 MHz. Thus, the harmonics produced by the system clock causes interference with GSM channel No. 5. Further, additional interference is caused by other harmonics that may not necessarily be generated by the system clock, but rather by other clocks in the system and by spurious mixes of local oscillators. In heterodyne or super-heterodyne receivers, local oscillators, mixers, and various digital clocks emit a wide range of harmonics that interfere with various GSM channels.
One approach to solve this problem in prior art systems is to add shielding to the xe2x80x9cnoisyxe2x80x9d components of the system so that the harmonic energy does not substantially escape or enter the receiver circuitry. This involves using various metal enclosures and screens, and may include specialized printed circuit board design. This, however, adds significant weight and increases the size of the device. Because weight and size are extremely important considerations in the design of a GSM system, such as a cellular telephone, the disadvantages of this approach are apparent. Further, use of shielding increases the cost of the device, which is usually sold in a cost-competitive market. Other approaches use additional receiver data collections to take a xe2x80x9csamplexe2x80x9d of the interference during a time when the input to the receiver is blocked. The xe2x80x9cinterference dataxe2x80x9d is then quantified and subtracted from the subsequent data collections. This approach, however, is disadvantageous because additional data collections consume additional power.
Reduction of internal interference in a radio-frequency (RF) receiver may be accomplished by collecting data during a predetermined receive time slot of a frame. A predetermined portion of the data collected is inspected, and a bias data value is calculated. The bias value is attributable to interference signals caused by the internal interference. The bias data value is accumulated, and over time, a running average of the bias data values is determined. A portion of the running average is subtracted from the sample stream of data to provide output data corresponding to the external RF signals absent the internal interference.